1. Field of the Invention
The present invention relates to an optical amplifier employed in optical fiber communications, optical measurements, or laser processing, and to the excitation light source for the laser thereof, the present invention providing a means for obtaining a higher output product.
The present specification is based on a patent application filed in Japan (Japanese Patent Application No. Hei 11-33524), a portion of the aforementioned application being incorporated herein by reference.
2. Description of the Related Art
FIG. 10A is a schematic structural diagram showing an example of a conventional long distance optical fiber communications system. As shown in this figure, in this conventional communications system, it is necessary to perform regenerative repetition every several tens km along optical fiber 3 joining transmitter 1 and receiver 2, in which an optical/electrical conversion, and then an electrical/optical conversion, are performed on the optical signal by regenerative repeaters 4, 4.
FIG. 10B shows an example of the structure of this regenerative repeater 4. The optical signal from optical fiber 3 on the transmitter 1 side undergoes optical/electrical conversation by passing through light detector 5, waveform shaping circuit 6, laser driver 7 and laser 8. The signal then undergoes electrical/optical conversion, and is sent to optical fiber 3 on the receiver 2 side.
Due to progress in optical technology, it has become possible to obtain a high output laser inexpensively in recent years. Thus, as shown in FIG. 11A, an optical amplifier 10 is inserted along optical fiber 3, in between transmitter 1 and receiver 2, to realize a long distance optical fiber communications system in which the light signal is directly amplified by optical amplifier 10.
FIG. 11B shows an example of the structure of optical amplifier 10. In optical amplifier 10, a rare-earth doped optical fiber 11 is the active media in which the amplification is actually carried out. The optical signal is amplified by inputting excitation light from laser 14 (excitation light source) to rare-earth doped optical fiber 11, via an optical fiber 13 which is connected to optical multiplexing element 12 provided at the front of rare-earth doped optical fiber 11. This amplified optical signal is then output from rare-earth doped optical fiber 11.
Isolator 15, which is toward the rear of rare-earth doped optical fiber 11, is provided to stabilize the operation of laser 14 by preventing feedback light.
By realizing an optical amplifier in this way, the attenuated optical signal is directly amplified, so that transmission without regenerative repetition is possible, even in the case of transmission over several thousand kilometers.
In current optical fiber communications, a 1.5 xcexcm band amplifying erbium doped optical fiber known for its high efficiency is primarily used for rare-earth doped optical fiber 11.
The absorption spectrum of the rare-earth element for forming rare-earth doped optical fiber 11 will differ depending on the type of rare-earth element employed. For example, as shown in FIG. 12, an erbium doped optical fiber has absorption spectrums of a comparatively broad wavelength width near 980 nm and 1480 nm. Thus, in a 1.5 xcexcm band amplifying erbium doped optical fiber amplifier (denoted as xe2x80x9cEDFAxe2x80x9d hereinafter), a 1.5 xcexcm band optical signal typically can be amplified using excitation light near 0.98 xcexcm or 1.48 xcexcm.
Typically, a semiconductor laser is employed as a laser 14 for oscillating the excitation light. Of these, a Fabry-Perot semiconductor laser (referred to as a xe2x80x9cFabry-Perot laserxe2x80x9d, hereinafter) is mainly used in which power can be obtained relatively inexpensively.
On the other hand, a wavelength multiplex mode optical fiber communications system has been realized for multiplex transmission of signal lights having a plurality of wavelengths. Thus, it has become possible to further increase the amount of information which can be transmitted by one optical fiber.
When carrying out wavelength multiplex communications in the optical communications system shown in FIG. 11A, the output required of optical fiber amplifier 10 is greater than in the case where transmitting a single wavelength. For this reason, the power of the excitation light supplied from laser 14 is also required to be greater.
As a method for increasing the total power of the excitation light, a method may be considered in which the output of the laser is increased, for example. There is a limit to this approach, however, since the output of a typical laser is limited. As a result, sufficient effects cannot be obtained.
Thus, the following method may be considered.
Namely, a plurality of lasers is prepared. These lasers oscillate light in a wavelength band capable of exciting the rare-earth element in the rare-earth doped optical fiber, and have oscillation wavelengths which differ slightly from one another. The lights output from these lasers are multiplexed, and this multiplexed light is used as the excitation light.
For example, as shown in FIG. 12, the wavelength width in the excitation wavelength band around 1.48 xcexcm in an EDFA is on the order of 1.45xcx9c1.49 xcexcm, and the excitation wavelength width around 0.98 xcexcm is on the order of several nm. These excitation wavelength widths are comparatively broad. Thus, when a plural lights having different wavelengths respectively within this wavelength band are multiplexed, the total of the various powers of the light becomes the power of the excitation light.
In other words, when n lasers are prepared, it is theoretically possible to obtain n-fold greater power as compared to the case where employing just one laser (assuming no loss when multiplexing, etc.).
However, as shown in FIG. 13, numerous vertical modes are present in the oscillation wavelengths of the Fabry-Perot laser that is typically used as laser 14. These oscillation wavelengths have a broad wavelength width on the order of 15xcx9c20 nm. In general, multiplexing a plurality of lights having this type of broad wavelength width is difficult. However, polarized waves are present in the light output from a Fabry-Perot laser. For this reason, a method is performed for obtaining excitation light of a two-fold greater power by multiplexing two polarized waves that are perpendicular to one another. In theory, however, in this method as well, it is not possible to obtain excitation light having a power in excess of two-fold greater than normal.
Moreover, when lights having too broad oscillation wavelength widths are multiplexed, the width of the wavelength band of the multiplexed light is crowded out of the excitation wavelength band, decreasing efficacy as excitation light.
Accordingly, the following method may be considered.
First, as shown in FIG. 14A, a reflecting element 20b (external resonator) for reflecting light in a specific narrow wavelength band at a low reflection coefficient is attached to the rear of Fabry-Perot laser 20a. 
The combination of Fabry-Perot laser 20a and reflecting element 20b is formed to have a structure identical to the so-called Distributed Bragg Reflector laser (DBR laser), in which one of the reflecting surfaces of a Fabry-Perot laser is substituted with a DBR (distributed Bragg reflector). In other words, a laser oscillating element 20 is formed which oscillates only light in the wavelength band that is selectively reflected by reflecting element 20b. 
As a result, the light obtained via reflecting element 20b is rendered into a narrow spectrum as shown in FIG. 14B.
FIG. 15A shows an example of a light source for multiple wavelength excitation in wavelength multiplex mode employing a laser oscillating element consisting of this type of Fabry-Perot laser and reflecting element, and shows the design of an optical amplifier incorporating the aforementioned light source. FIG. 15A is a schematic structural diagram showing an example in which this optical amplifier is employed in an optical communications system using a wavelength multiplex transmission mode.
Namely, n laser oscillating elements as described above are prepared.
In the figure, the numeral 21 is a laser oscillating element for oscillating light in a narrow band region centered on wavelength 1. Laser oscillating element 21 is composed of Fabry-Perot laser 21a and reflecting element 21b. Similarly, laser oscillating element 22 for oscillating light centered on wavelength xcex2 is composed of Fabry-Perot laser 22a and reflecting element 22b. The nth laser oscillating element 23 consists of Fabry-Perot laser 23a and reflecting element 23b, and oscillates light centered on wavelength xcexn.
The oscillation wavelengths xcex1, xcex2, . . . xcexn of these laser oscillating elements 21, 22 . . . 23 are set so as to differ at suitable wavelength intervals.
When employing an erbium doped optical fiber as rare-earth doped optical fiber 11, a plurality of laser oscillating elements are prepared having oscillation wavelengths which differ at specific intervals in the 980 nm and 148 nm excitation wavelength bands, to form an optical amplifier as shown in FIG. 15A.
In this multiple wavelength excitation light source, the lights oscillated from these laser oscillating elements 21, 22 . . . 23 are input respectively to the input terminals 24a, 24b . . . 24c of optical multiplexing element 24, multiplexed at multiplexing element 24, and then output from output terminal 24d. In this multiplexed light, as shown in FIG. 15B, a wavelength spectrum is obtained in which a plurality of peaks xcex1, xcex2 . . . xcexn are aligned. The peaks xcex1, xcex2, . . . xcexn have narrow wavelength widths respectively. The power of this multiplexed light is the sum of the respective powers of these peaks.
This multiplexed light is input to rare-earth doped fiber 11 via optical multiplexing element 12. As a result, the optical signal of xcex1xe2x80x2, xcex2xe2x80x2, . . . xcexnxe2x80x2 which was propagated through transmission optical fiber 3 employing a wavelength multiplexing mode and input to rare-earth doped optical fiber 11 is amplified due to the excitation effect of this multiplexed light.
However, the following problems are present in the method shown in FIG. 15A for multiplexing light having a plurality of different wavelengths.
The first problem is that a high wavelength accuracy is required of reflecting elements 21b, 22b . . . 23b. 
This is because the oscillation wavelength must be made to strictly correspond to each laser oscillating element 21, 22, . . . 23. Further, when the wavelength accuracy of reflecting element 21b is low, the oscillation wavelength is not stabilized, so that multiplexed light having a large power cannot be obtained.
For example, when using a 980 nm band as the excitation wavelength band when forming an EDFA, a plurality of reflecting elements 21b, . . . 23b having the reflecting wavelength characteristics 977 nm, 978.5 nm, 980 nm, and 981.5 nm, respectively, are prepared, and the oscillation wavelengths of respective laser oscillating elements 21 , . . . 23 must be matched. In other words, since the oscillation wavelengths of the plurality of laser oscillating elements 21, . . . 23 are adjacent, considerably high accuracy is required of reflecting elements 21b, . . . 23b. 
In addition to the first problem, there is a second problem in that the oscillation wavelengths of laser oscillating elements 21, . . . 23 must match the transmission wavelength characteristics of optical multiplexing element 24 in which these oscillation wavelengths are multiplexed.
If they do not match, then the lights oscillated from laser oscillating elements 21, . . . 23 are damped at optical multiplexing element 24, and are not transmitted to the output side of optical multiplexing element 24 with high efficiency.
Moreover, this phenomenon is a relative one. Even if the above-described first problem is resolved and the respective oscillation wavelengths from laser oscillating elements 21, . . . 23 are stable, if the transmission wavelength of optical multiplexing element 24 varies, then the light is damped at optical multiplexing element 24, and the power of the multiplexed light changes greatly.
However, the transmission wavelength characteristics of optical multiplexing element 24 are highly dependent on temperature.
For example, optical multiplexing element 24 is assumed to have a temperature dependence of 0.013 nm/xc2x0C. In the case where temperature compensation is not carried out for optical multiplexing element 24, when the employed temperature environment changes 50xc2x0 C. in the temperature range, then the transmission wavelength parallel shifts by just 0.65 nm on the wavelength axis. In this case, the transmission wavelength of optical multiplexing element 24 is at cross purposes with the oscillation wavelength of laser oscillating elements 21, . . . 23, and the transmission loss at optical multiplexing element 24 increases sharply. Accordingly, strict temperature compensation for optical multiplexing element 24 has been required.
On the other hand, when temperature control is not carried out for the oscillation wavelength of Fabry-Perot lasers 21a, . . . 23a, it is known that when, for example, the temperature changes from 0xc2x0 C. to 40xc2x0 C., the central wavelength changes by 10 nm or more. Accordingly, strict temperature compensation for Fabry-Perot lasers 21a, . . . 23a is required in order to make the reflected wavelength from reflecting elements 21b, . . . , 23b and the transmitted wavelength from optical multiplexing element 24 match the oscillation wavelength of Fabry-Perot lasers 21a, . . . 23a. 
This type of high output is demanded not only in the optical communications field, but also in optical amplifiers and their laser excitation light sources which are used in optical measurements, laser processing and the like.
The present invention relates to a multiple wavelength excitation optical multiplexing device capable of outputting high output excitation light, a light source incorporating this device, and an optical amplifier incorporating this device, and is directed to the provision of a device in which the characteristics do not readily change in response to temperature variations.
In addition, the present invention is also directed to the provision of a device in which the structural parts are not required to have as high a wavelength accuracy as demanded in the conventional art.
It is also the present invention""s objective to provide a multiple wavelength excitation optical multiplexing device, a light source incorporating this device, and an optical amplifier incorporating this device which are capable of reducing temperature control for the structural parts.
The following means of resolution are provided in the present invention.
Namely, a multiple wavelength excitation optical multiplexing device is formed characterized in the provision of an optical multiplexing element that has a plurality of input terminals connected to lasers and functions to multiplex a plurality of lights that have different characteristics; and a reflecting element inserted near the output side of the output terminal of the optical multiplexing element, and functioning to reflect the light multiplexed at the optical multiplexing element at a low reflection coefficient.
In addition, a multiple wavelength excitation light source. is formed characterized in that respective lasers are connected to the input terminals of the optical multiplexing elements in this multiple wavelength excitation optical multiplexing device. In addition, an optical amplifier is formed by incorporating this multiple wavelength excitation light source.
The following effects can be obtained in the present invention""s multiple wavelength excitation optical multiplexing device and multiple wavelength excitation light source.
Namely, the respective oscillation wavelengths of the lasers are determined by the optical multiplexing element. Therefore, even if the transmission characteristics of the optical. multiplexing element vary due to temperature changes, the oscillation wavelength of the laser varies in concert with this change. As a result, the power of the multiplexed light obtained via the optical multiplexing element does not readily change due to temperature variations, so that it becomes possible to provide a stable high output excitation light.
Accordingly, temperature control of the optical multiplexing element can be relaxed, and structural elements such as the reflecting elements, etc., are not required to. have as high a wavelength accuracy. Moreover, there are a fewer number of parts as compared to the conventional design in which one reflecting element was employed per laser. Thus, it is possible to reduce loss and part costs, so that the present invention is economical.
Further, by incorporating the present invention""s multiple wavelength excitation light source in an optical amplifier, it is possible to obtain a high output optical amplifier suitable for use in a wavelength multiplex transmission mode optical communications system.